Firm forced to engage Republican associate (7)
I believe the answer is:
comrade
'associate' is the definition.
(I know that associate can be written as comrade)
'firm forced to engage republican' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'forced' becomes 'made' (both can mean coerced).
'to engage' means one lot of letters goes inside another (some letters get engaged in others).
'republican' becomes 'R' (abbreviation).
'made' placed around 'r' is 'mrade'.
'co'+'mrade'='COMRADE'
(Other definitions for comrade that I've seen before include "Pal" , "Ally" , "Fellow left-winger" , "Colleague" , "Fellow soldier" .)